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the sample are treated with 5-6 c.c. of sulphuric acid and 15-20 c.c. of
nitric acid, the liquid being diluted to 250-300 c.c. and electrolysed to
eliminate the copper (see i). The electrodes are then removed and washed
with water, and the solution evaporated until the nitric acid is completely
expelled. If the anode appears brown, it is left for some time immersed in
the evaporating liquid, the peroxides deposited on it being caused to dis-
solve by addition of a few drops of hydrogen peroxide. When cold the
residue is dissolved in water and the solution treated in the hot with hydro-
gen sulphide to precipitate the arsenic, lead, antimony, etc. The precipitate
is filtered off and washed, and the nitrate boiled to expel the hydrogen
sulphide, treated with 1-2 c.c. of hydrogen peroxide, boiled and rendered
alkaline with ammonia to precipitate the iron. When considerable quan-
tities of iron are present, it is advisable to repeat this precipitation after
solution of the precipitate in a little hot, dilute sulphuric acid. The feme
hydroxide separated is washed, dried and ignited ; it may be weighed
directly or dissolved in a little dilute sulphuric acid and estimated electro-
lytically (see p. 200).

The filtrate, or the mixed filtrates from the two precipitations, are used
for the electrolytic determination of the nickel and zinc. First, 30-40 c.c.
of ammonia and a few crystals of hydroxylamine sulphate are added and
the nickel then deposited, the zinc being determined in the residual solution
(see Analysis of Argentan).

When determination of the zinc is not required, the nickel may be pre-
cipitated directly in the iron-free alkaline solution by alcoholic dimethyl-
glyoxime solution (see Gravimetric Analysis of Argentan).

9.  Determination of the Silver.2-5 grains of the. sample are dis-
solved in 100 c.c. of nitric acid diluted with an equal volume of water, the
solution being boiled to expel nitrous vapours and, without filtering, heated
to 95 and treated with a few drops of hydrochloric acid to precipitate the
silver.   The liquid is heated on the water-bath to separate the precipitate,
which is filtered off and washed with hot water, the silver being determined
in this impure silver chloride either electrolytically or gravimetrically.

(a]  ELECTROLYTIC METHOD.   The precipitate is dissolved in the hot
in 120 c.c. of 10% potassium cyanide solution, then diluted to 300 c.c.
and electrolysed with a current of o-i ampere.

(b)  GRAVIMETRIC METHOD.   The silver chloride is dissolved in ammonia
by digestion in the hot for some time, the solution being filtered and acidified
with nitric acid to reprecipitate the silver chloride, which is weighed as

10.  Determination of the Total Oxygen.- The oxygen present in
commercial copper occurs largely as cuprous oxide and partly in combina-
tion with extraneous metals.    It is determined by causing it to combine
with hydrogen at a high temperature and estimating the water formed.
A condition essential to accuracy is that the hydrogen must be perfectly
dry and free from oxygen.

The hydrogen, made in a Kipp apparatus from pure zinc and dilute
sulphuric acid, is purified by passing it successively through alkaline pyro-
gallol, permanganate solution and cone, sulphuric acid. Any traces of1 In case the copper has been deposited twice, the solution from the second